dt-bindings: Document common property for daisy-chained devices

Many serially-attached GPIO and IIO devices are daisy-chainable.

Examples for GPIO devices are Maxim MAX3191x and TI SN65HVS88x:
https://datasheets.maximintegrated.com/en/ds/MAX31913.pdf
http://www.ti.com/lit/ds/symlink/sn65hvs880.pdf

Examples for IIO devices are TI DAC128S085 and TI DAC161S055:
http://www.ti.com/lit/ds/symlink/dac128s085.pdf
http://www.ti.com/lit/ds/symlink/dac161s055.pdf

We already have drivers for daisy-chainable devices in the tree but
their devicetree bindings are somewhat inconsistent and ill-named:

The gpio-74x164.c driver uses "registers-number" to convey the
number of devices in the daisy-chain. (Sans vendor prefix,
multiple vendors sell compatible versions of this chip.)

The gpio-pisosr.c driver takes a different approach and calculates
the number of devices in the daisy-chain by dividing the common
"ngpios" property (Documentation/devicetree/bindings/gpio/gpio.txt)
by 8 (which assumes that each chip has 8 inputs).

Let's standardize on a common "#daisy-chained-devices" property.
That name was chosen because it's the term most frequently used in
datasheets. (A less frequently used synonym is "cascaded devices".)

Signed-off-by: Lukas Wunner <[e-mail]>
Acked-by: Jonathan Cameron <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/common-properties.txt

···
 Common properties
+=================
+
+Endianness
+----------
 
 The Devicetree Specification does not define any properties related to hardware
 byteswapping, but endianness issues show up frequently in porting Linux to
···
 	      reg = <0x40031000 0x1000>;
 	      ...
 	      little-endian;
+};
+
+Daisy-chained devices
+---------------------
+
+Many serially-attached GPIO and IIO devices are daisy-chainable.  To the
+host controller, a daisy-chain appears as a single device, but the number
+of inputs and outputs it provides is the sum of inputs and outputs provided
+by all of its devices.  The driver needs to know how many devices the
+daisy-chain comprises to determine the amount of data exchanged, how many
+inputs and outputs to register and so on.
+
+Optional properties:
+ - #daisy-chained-devices: Number of devices in the daisy-chain (default is 1).
+
+Example:
+gpio@0 {
+	      compatible = "name";
+	      reg = <0>;
+	      gpio-controller;
+	      #gpio-cells = <2>;
+	      #daisy-chained-devices = <3>;
 };